Due to its excellent waterproofing and strong adhesive properties, asphalt materials have been widely used in highway, airport, and building construction. As a thermoplastic material, the mechanical properties of asphalt greatly change with temperature and time of loading. These materials are commonly manufactured at a higher temperature range of 280-320° F. (138-160 ° C.) for better workability and controllable quality. During the material manufacturing, a great amount of greenhouse gas and other emissions are produced, which potentially contributed to global warming and local environment deterioration. Due to the smoke and odor in material production, the applications of asphalt materials are often limited to night construction in some urban areas. Moreover, the aging process of asphalt will be accelerated at a high temperature, which may negatively affect its life cycle performance.
Warm Mix Asphalt (WMA) refers to asphalt mixes that are produced at lower temperatures than HMA. There are typically three categories in which to characterize the production of asphalt mixtures: Cold Mix; Warm Mix Asphalt; and Hot Mix Asphalt. These classifications are based on the temperature at which these mixtures are produced. WMA mixtures are commonly produced at temperatures 40-50° C. (104-122° F.) lower than HMA, which are produced at about 100-140° C. (212-284° F.).
There are many environmental benefits to producing asphalt as a WMA. Since asphalt is a petroleum based product, when heated to high temperatures it yields CO2 as a by-product. WMA can drastically decrease these emissions, because at lower temperatures the WMA mixture produced in the drum and placed in a construction site requires less gas/fuel consumption and releases less smoke/odor, respectively. The working conditions are improved because of the decrease in fumes and emissions for the workers.
The asphalt mix when created as a WMA becomes more versatile. Many benefits arise from WMA mixes during paving. Improved workability because of lower viscosities, the ability for cold weather paving because of the change in temperature between the road and the asphalt over time is decreased, reduced pavement cooling time, and the ability to pave longer distances because of lower initial temperature.
Within the WMA category there were three different ways to classify the technology: foaming technique; organic/wax additive; and chemical additives. In recent years, some emerging WMA technologies and mechanisms have been proposed.
Foaming processes, as an excellent example of green technologies, save energy cost and reduce CO2 emission. They are also used in recycling asphalt pavements. The main research in foaming processes of asphalt materials focuses on the performance and workability test. Due to a lower production temperature, the moisture within asphalt may not be completely expelled out and thus increase potential for moisture damages such as stripping and segregation.
Foaming process was developed in asphalt industry for soil stabilization around 50 years ago. In recent years, due to its lower energy consumption and emissions, highway engineering industry started to use it in cold in-place recycling pavement, cold mix asphalt, and warm mix asphalt construction.
The traditional foamed asphalt is called expanded asphalt. Expanded asphalt is produced when water is injected into the hot asphalt binder. The water is turned into vapor and is trapped in numerous tiny bubbles in the asphalt, causing spontaneous foaming, causing the volume of asphalt to increase greatly. However, the foam dissipates in a very short time, often less than one minute, so that asphalt materials have to be manufactured within a small time window while the asphalt binder is still in its foamed state.
Foaming processes significantly increase the volume of asphalt binder with large surface area in the unit volume. When asphalt mixes with aggregate, a strong coating with high shear strength of the mix can be generated. In addition, because foamed asphalt is flexible and has much larger volume, the workability is considerably improved, so that asphalt materials manufacturing can be operated at a lower temperature within a shorter time. The optimum asphalt content of a foamed asphalt mix can be lower than that for the non-foamed one. Therefore, the following advantages of foamed asphalt are well documented as: reduced binder cost, saving in time, energy conservation, environmental benignancy, better workability, and broad applicability. In addition, because water is only used as an agent during production, and will be expelled in compaction, the chemical properties of asphalt are not changed and thus the existing engineering specifications are still applicable. Moreover, because aging effects are reduced at lower temperatures, the long-term performance of asphalt materials will be improved.
However, in the traditional foaming process, the volume of asphalt binder significantly changes within the short process window, which produces difficulties in quality control. In recent years, emerging foaming processes have been invented to prolong the foaming process window and therefore extend the applications to massive construction and production. For example, zeolite, a water bearing additive, is added to asphalt binder to reduce the production temperature and improve the workability.
In construction, with the addition of up to 0.3% zeolite in weight of total asphalt mixtures, asphalt production temperature can be reduced to 50-70° F. lower than that of traditional hot mix, so that up to 40% energy savings and 60% emission reduction can be achieved. Low energy asphalt (LEA) was introduced in 2005 in France. This new process uses sequential mixing of the asphalt binder with a chemical additive and coarse aggregate at high temperature, followed by adding wet sand to create a foaming action. Since the fine sand carries the most moisture, they consume the most amount of energy to dry and lower down the production temperature. The desired mix discharge temperature is as low as 203° F., and thus causes a 40-55% reduction in energy consumption.
As a green construction technology, foaming based WMA has been used in recycling asphalt pavement and new construction, and demonstrates great potential to: Reduce energy consumption and cost; Reduce emissions such as carbon dioxide, blue smoke, and odor; Reduce binder oxidation prolonging the life of asphalt materials; Improve compactability to assure the in-place density of asphalt pavements; and Reduce the life cycle cost of asphalt materials.
As a result, a lot of construction projects, which are impossible for traditional hot mix asphalt, can be done with these foaming processes, such as projects requiring long haul distances, constructed in lower temperature weather or seasons, and sited in urban area. However, the physics and chemistry of this empirical engineering practice has not been fully understood, and some problems have been observed in construction.